SCADA - Supervisory Control And Data Acquisitionmasek/7680_notes.pdf · 2017. 1. 25. · SCADA - Supervisory Control And Data Acquisition ENGI-7680 Winter 2016. DAC ... The process

DAC - Digital to Analog ConversionADC - Analog to Digital Conversion

RS485 - Modbus

SCADA - Supervisory Control And DataAcquisition

ENGI-7680

Winter 2016


RS485 - Modbus

Outline

1 DAC - Digital to Analog Conversion

2 ADC - Analog to Digital Conversion

3 RS485 - Modbus


RS485 - Modbus

Digital To Analog Converter

To output analog voltage from a PC or µC, the numerical value(integer) must be converted to the analog voltage equivalent bya D/A converter. Analog outputs are much simpler than analoginputs. This process is very fast, and does not experience thetiming problems of sampling and conversion with analog inputs.However, analog outputs are still subject to quantization errors.


RS485 - Modbus

Weighted Resistor Summing DAC

Develop an analytical model for 3-bit DAC output in terms of theresistances, reference voltage and the switch positions.


RS485 - Modbus

Ladder Resistor Network DAC

2R

2R

2R

R

Vr

A1

A0

Vo

V o

R 1

R 2

R 3

R4

R5

R6

21

A 2

3

21

A 1

3

21

A 0

3

+-G

1

2 R

2 R

2 R

2R

RR

S W

S W

S W

BA

T

G N D

Ex: Develop a model for 2-bit DAC output, via superposition.Prelab L1-1: Develop an analytical model for 3-bit DAC outputin terms of the R-2R resistances, reference voltages and theswitch positions.Prelab L1-2: Develop a model for the switch currents.Prelab L1-3: Install Arduino IDE ∼masek/arduino01.pdf


RS485 - Modbus

Ladder Resistor Network DAC - cont’d

Solution to Prelab L1-1:

v0Vref

=R22 + 5R1R2 + 2R

21

4R22 + 7R1R2 + 2R21

D2 +

+R2(R2 + 2R1)

4R22 + 7R1R2 + 2R21

D1 +

+R22

4R22 + 7R1R2 + 2R21

D0


RS485 - Modbus

Ladder Resistor Network DAC - cont’d

Test of the solution to Prelab L1-1:

R2 = 2R1

v0Vref

=D22

+D14

+D08


RS485 - Modbus

Uncertainty

Maximum uncertainty1:

∆MAXVDAC =

∣∣∣∣∂VDAC∂R1∣∣∣∣∆R1 + ∣∣∣∣∂VDAC∂R2

∣∣∣∣∆R2 + . . .+ ∣∣∣∣∂VDAC∂RN∣∣∣∣∆RN

Probable uncertainty2:

∆PROBVDAC =

√(∂VDAC∂R1

∆R1

)2+

(∂VDAC∂R2

∆R2

)2+ . . .+

(∂VDAC∂RN

∆RN

)2Assignment A1-1,2: Calculate DAC uncertainty for both, (1)3-bit Weighted Resistor Summing DAC and (2) 3-bit LadderResistor Network DAC, assuming 1% tolerances.

1represents the worst case uncertainty2represents a more realistic uncertainty


RS485 - Modbus

A1-1 Solution

Vo = −(RR2

D2 +RR4

D1 +RR8

D0)VR

∆MAXVo

∣∣∣D0=D1=D2=1

=

∣∣∣∣ RR2 VR∣∣∣∣ ∆R2R2 +

∣∣∣∣ RR4 VR∣∣∣∣ ∆R4R4 +

∣∣∣∣ RR8 VR∣∣∣∣ ∆R8R8

∆MAXVo = (12× 1% + 1

4× 1% + 1

8× 1%)VR

∆MAXVo =78× 1%VR=̇0.87%VR


RS485 - Modbus

A1-2 Solution

∆MAXVo

∣∣∣D0=D1=D2=1

=

∣∣∣∣ R22(7R2 + 4R1)(4R22 + 7R1R2 + 2R21)2 VR∣∣∣∣∆R1 +

+

∣∣∣∣ R1R2(7R2 + 4R1)(4R22 + 7R1R2 + 2R21)2 VR∣∣∣∣∆R2

∆MAXVo

∣∣∣R2=2R1

= (9

128×

∆R1R1

+9

256×

∆R2R1

)VR

∆MAXVo

∣∣∣R2=2R1

= (9

128×

∆R1R1

+9

128×

∆R22R1

)VR

∆MAXVo =9

64× 1%VR=̇0.14%VR

The three bit R-2R DAC exhibits over six times less uncertaintyin comparison to the three bit weighted resistor DAC.


RS485 - Modbus

PWM to Voltage - filter design

Required Attenuation in terms of the ripple voltage:

AdB = 20× logVRIP

VPWM(1)

Required Attenuation in terms of the slope S (-20, -40, ...):

AdB = S × logfPWMf3dB

(2)

f3dB = fPWM × 10−AdB

S (3)

RC filter:f3dB =

12π × RF × CF

(4)


RS485 - Modbus

PWM to Voltage - filter design cont’d


RS485 - Modbus

PWM to Voltage - IC

MILPITAS, CA - August 12, 2014 - Linear TechnologyCorporation introduces the LTC2645, a quad-channel12-bit/10-bit/8-bit PWM-to-voltage output digital-to-analogconverter (DAC) with 10ppm/Â◦C reference. These partsconvert PWM input signals to 12-bit accurate, stable, bufferedvoltage outputs in less than 8 microseconds, eliminating theripple and delay typically associated with analog filters. ...

The LTC2645 measures the period and pulse width of the PWM inputsignals and updates the DACs after each PWM input rising edge,accepting PWM input frequencies from 30Hz up to 100kHz. AnIDLSEL pin provides flexibility to set the outputs to idle at zero or fullscale, power-down with high-impedance output, or hold the previousstate indefinitely in response to an idle PWM input. This convenientmode has the advantage over analog filter implementations, whichrequire the PWM to run continuously.


RS485 - Modbus

Analog to Digital Conversion - definitions

To input an analog voltage into a PC or µcontroller, thecontinuous voltage value must be first sampled and thenconverted to a numerical value by an A/D converter.

The process of sampling the data is not instantaneous, soeach sample has a start and stop time. The time requiredto acquire the sample is called the sampling time ts.A/D converters can only acquire a limited number ofsamples per second. The time between samples is calledthe sampling period TS, and the inverse of the samplingperiod is the sampling frequency/rate. Ts < TSThe maximum Vmax and minimum Vmin readable voltagesare a function of the control hardware such as 0V to 5V.The number of bits of the A/D converter is the number ofbits in the result word. If the A/D converter is 8 bit then theresult can read up to 256 different voltage levels.


RS485 - Modbus

Sampling

Sample & Hold (S/H) circuitry takes a snapshot of the inputsignal and holds the value for the A/D converter to have a stablesignal. Often a FET switch connects capacitor to buffered inputat the beginning of every sample period. The capacitor thenholds the voltage value sampled until a new sample is acquired.This voltage slowly decreases over time despite of the highimpedance output buffer. It is then necessary to perform theA/D conversion quickly in a short period of time.


RS485 - Modbus

S&H Chopper Circuit


RS485 - Modbus

Flash ADC

consist of a set of comparators and reference voltages,and a digital encodertypically 4 to 8 bit designs due to the required number ofcomparatorsfast scanning ratestypically used in TV, fast measurement instrumentsoften a bubble error correction is applied prior to digitalencoding


RS485 - Modbus

Successive Approximation ADC

> 7.5> 7

< 6.5> 6

> 5.5< 5

< 4.54

> 3.5> 3

< 2.5< 2

> 1.5< 1

< 0.5MSB 22 21 LSB 20 ±0.5V = LSB/2


RS485 - Modbus

Timer/Counter based ADC’sRC network with a comparator

VC = VR(1− e−tτ )

τ = RC

Charging a capacitor method is a simple time-interval basedtechnique, however, the accuracy is low due to non-linearity,and VR / RC stability.


RS485 - Modbus

Timer/Counter based ADC’s - cont’dDual Slope Integrating ADC

applications that do not require high speed samplingimmune to non-linearity of the integratorcan be very accurate provided a longer time of conversionis acceptable

An integrator is the core element in the dual-slope ADC. Azeroed integrator (Vint = 0) is first connected to the measuredvoltage VS&H for a fixed time duration (run-up time T1 =const).This causes the integrator to wind up to a certain voltage, sayV uint . Then a fixed reference voltage VR of the opposite polaritythan VS&H is connected to the integrator which causes theintegrator to back-integrate from V uint to zero voltage. Thisphase is often referred to as run-down phase of time durationT2.


RS485 - Modbus

Timer/Counter based ADC’s - cont’dDual Slope Integrating ADC - SUMMARY

Two voltage signal integration phases:1 integration of the input voltage VS&H over a constant time

period T1 starting at zero (0V)2 (back) integration of the constant reference voltage VR over

a variable time T2 until zero level crossing occurs (0V)

Using a ratiometric method, the non-linearity and drift ofRC is not critical as was the case of RC network ADC or asingle slope ADC.

VS&H = VR ×T2T1

(5)


RS485 - Modbus

Assignment II.

1 Specify all components in the dual slope ADC circuit anddefine the timing sequence of control lines S-H, RES, INT,DEINT.

2 Simulate the analog to digital conversion using SPICEtransient analysis and provide these waveforms: S-H(t),RES(t), INT(t), DEINT(t), VINT (t), COMP(t).

3 Propose a solution to minimize the transient ’spikes’ at theintegrator’s input.

4 Derive an expression for the measured voltage in terms ofthe design parameters and the measured time ofde-integration.


RS485 - Modbus

Assignment II. - cont’d

Four digital outputs are required to control the conversionprocess assuming only uni-polar voltage being measured. Onedigital output controls S&H operation, two digital outputs controlswitching of VS&H and VR to the integrator input, and one digitaloutput is used to discharge the capacitor at the beginning ofeach conversion cycle.

LTspice: ∼masek/dual-slope-ADC.asc


RS485 - Modbus

Assignment II. - solution


RS485 - Modbus

RS232 and RS485

RS232: data terminal equipment←→ data communicationequipmentlow speed of kbpslow cable length (50 feet)poor immunity for noise on the signal linesshifts in the ground level cause problemsRS485: multiple nodes communicate with each othersignals are floating and each signal is transmitted over aSig+ line and a Sig- linereceiver compares the voltage difference between bothlines, instead of the absolute voltage level on a signal line.for high noise immunity, a combination of twisting andshielding is useddifferential signal lines allow higher bit rates of mbpstermination resitors to avoid reflections on longer cables


RS485 - Modbus

RS485

Older RS485 transceivers with an input resistance of 12kΩ allowed 32 devices on the network.Modern RS485 transceivers allow 256 devices on thenetworkRS485 repeaters are available (several thousand nodes,spanning multiple kilometers)RS485 drivers automatically return to high impedancetri-state within a few microseconds after the data (0 bit) hasbeen sent.


RS485 - Modbus

Assignment due date March 3, 2015

Generate MODBUS response to the following requests; for thewriting commands (Force/Preset) update the memory imagebelow and indicate the corresponding column number; updateonly the fields that undergo changes. Additional informationmay be found in the Modbus Protocol Reference Guide.


RS485 - Modbus

Assignment - cont’d

REQ: 01 02 00 02 00 02 XX XX

RES:

REQ: 01 05 00 0B FF 00 XX XX

RES:

REQ: 01 08 00 00 A8 57 XX XX

RES:

REQ: 01 03 00 02 00 03 XX XX

RES:

REQ: 01 04 00 01 00 05 XX XX

RES:

REQ: 01 06 00 04 1C B5 XX XX

RES:

REQ: 01 10 00 02 00 02 04 00 0B 05 07 XX XX

RES:

REQ: 01 01 00 0A 00 12 XX XX

RES:

REQ: 01 03 00 02 00 02 XX XX

RES:


RS485 - Modbus

Assignment - cont’d

10016 0000 0000

10015 0000 0001

10014 0000 0001

10013 0000 0001

10012 0000 0000

10011 0000 0001

discrete 10010 0000 0001inputs 10009 0000 0000

10008 0000 0000

10007 0000 0001

10006 0000 0000

10005 0000 0001

10004 0000 0001

10003 0000 0001

10002 0000 0000

10001 0000 0001

00016 0000 0001

00015 0000 0001

00014 0000 0000

00013 0000 0001

00012 0000 0000

00011 0000 0001

00010 0000 0001

00009 0000 0001

coils 00008 0000 000000007 0000 0001

00006 0000 0000

00005 0000 0001

00004 0000 0000

00003 0000 0000

00002 0000 0001

00001 0000 0001


RS485 - Modbus

Assignment - cont’d1111 1101

40008 0100 0100

1010 0101

40007 0100 0100

0110 1001

holding 40006 1111 1101registers 1011 1100

40005 0110 1001

1011 1100

40004 0100 0100

0110 1001

40003 0000 0000

1010 0101

40002 1111 1101

0100 0100

40001 1011 1100

0000 0000

30008 1011 1100

0000 0000

30007 1010 0101

0100 0100

30006 1111 1101

1011 1100

input 30005 0110 1001registers 1011 1100

30004 0000 0000

1010 0101

30003 1111 1101

0100 0100

30002 0000 0000

0100 0100

30001 1010 0101

Practical Industrial Data Networks: Design, Installation and

Troubleshooting

Chapter 6

Fiber Optics Overview

(I)

Feb. 10th 2011

http://www.mun.ca/

Chapter 6: Fiber optics overviewPractical Industrial Data Networks: Design, Installation and Troubleshooting

Objectives

• List of main features of fiber optics cables

• Fix problems with:

• Splicing

• Laser and LED transmitters

• Driver incompatibility

• Incorrect bending radius in installation

• Shock and other installation issues

• Interface to cable connectors

2

http://www.mun.ca/


6.1 Introduction

3

What’s an optical fiber?

Why is optical fiber?

How does it work?

http://www.mun.ca/


What’s an optical fiber?

4

Sir Charles Kuen Kao (1933 -)

"Father of Fiber Optic Communications”

Awarded half of the 2009 Nobel Prize

in Physics for "groundbreaking

achievements concerning the

transmission of light in fibers

for optical communication”.

http://www.mun.ca/http://upload.wikimedia.org/wikipedia/commons/4/49/Fibreoptic.jpg


Why is optical fiber?

• Waveguide of light

• Light signals are impervious from electromagnetic interference (EMI)

• Light signals do not interfere with other signals

• Much wider, flat bandwidth than coaxial cables

• Much lower attenuation

• Available in high voltage areas

• Much thinner and lighter than copper cable

• Greater data security than copper cable

• More abundant resources of material

5

http://www.mun.ca/


How does it work?

6

Total internal reflection:

Cladding

Core

n1

n2

Cladding n1

n1


Applications for optical fiber

7

Communication Sensing

Astronomy Illumination

http://www.mun.ca/


6.2 Fiber optical cable components (I)

8

Core

Cladding

Inner coating

Outer coating

400mm

250mm125mm

8.5 mm (single mode fiber)50 or 62.5 mm (multimode fiber)

Face view: Profile:

Core n2=1.49

Cladding n1=1.46

Polymeric coating

http://www.mun.ca/


6.2 Fiber optical cable components (II)

• Fiber core:

– Glass fiber (silica, SiO2)

– 8.5 mm diameter (single mode fiber)

– 50 mm or 62.5 mm diameter (multimode fiber)

– Light signal travels through it

• Cladding

– Manufactured with the core as a single unit

– Protective layer of core

– Typically 125 mm diameter

– Lower refractive index than that of core

9

http://www.mun.ca/


6.2 Fiber optical cable components (III)

• Fiber optic buffer– One or more layers of plastic surrounding the cladding

– Strengthen the cable, decreasing the likelihood of micro cracks

– Protect the core and cladding from potential invasion by water or other materials in environment

– Typically doubles the diameter of the fiber

– Tight or loss buffer

10

(a) Tightly Buffered Fiber (b) Loose Tube Structure

Hard Plastic Coating(900 mm)

Coated Fiber (250 mm)

Soft plastic bufferCoated Fibers(one or more)

Moisture Blocking Jelly

Hard Plastic Tube

http://www.mun.ca/


6.2 Fiber optical cable components (IV)

• Strength members

– Very tough material (steel, fiber glass or Kevlar)

– Provide tensile strength for cable

– Drawbacks: add weight of cable which may seriously damage the cable or equipment

• Cable sheath

– Outer casing of fiber optic cable

– Provides primary mechanical protection

11

http://www.mun.ca/


6.3 Fiber optic cable parameters (I)

• Attenuation

– Depends on the wavelength and the fiber construction

– Multimode fiber: 3-8 dB/km at 850 nm, 1-3 dB/km at 1330 nm

– Single mode fiber: 0.4-0.6 dB/km at 1330nm, 0.25-0.35 dB/km at 1550nm

• Diameter of core– Multimode fiber: 50 mm or 62.5 mm

• Multiple paths, difference in arrival time

– Single mode fiber: 8.5 mm• Difficult to align to system components and light sources

• Transmission speeds of 50Gbps and higher

12

http://www.mun.ca/


6.3 Fiber optic cable parameters (II)

• Wavelength

– Operates in one of three bands: 850 nm, 1330 nm, 1550 nm

– Shorter wavelengths have a greater attenuation

– Short haul systems use the 850 nm or 1330 nm with the multimode fiber and light emitting diode (LED)

– Long distance communications use 1550 nm wavelength with single mode fiber and laser sources

13

http://www.mun.ca/


6.3 Fiber optic cable parameters (III)• Bandwidth

– Given as the range of frequencies across which the output power is maintained within 3dB (about 50%) of the nominal output

– Quoted as the product of the bandwidth and distance

– 500 MHz–km: • 500 MHz of bandwidth is available over a distance of one kilometer

• or 100 MHz of bandwidth over five kilometers

14

http://www.mun.ca/


6.3 Fiber optic cable parameters (IV)• Dispersion

– Modal dispersion• Measured as nanoseconds of pulse spread per kilometer (ns/km)

• The duration of signal must be larger than the nanoseconds of a tail value

• The value imposes an upper limit on the bandwidth

• Between 15 and 30 ns/km for step-index fiber

• 20 ns/km modal dispersion yields a bandwidth of less than 50 Mbps

• No modal dispersion in signal mode fiber since only one mode is involved

15

http://www.mun.ca/


6.3 Fiber optic cable parameters (V)

– Chromatic dispersion• Only dispersion in single mode fiber

• Measured as the spread of the pulses in picoseconds for each nanometer of spectral spread of the pulse and for each kilometer traveled

• 3.5 ps/nm-km at 1300nm, 20 ps/nm-km at 1550 nm.

16

http://www.mun.ca/


6.4 Types of optical fiber (I)

• Step-index cable

– Cable with an abrupt change in refraction index, single-step

– Simplest, least expensive type of fiber optic cable, easiest to install

– The least attenuation for fiber-optic cable, 10-50 dB/km

– Transmission speeds up to 10 Mbps over 1 km

17

Light transmits in core

Refraction index of fiber

http://www.mun.ca/


6.4 Types of optical fiber (II)

18

• Graded-index cable

– Cable with a gradual change in refraction index

– Change occurs gradually and involves several layers each with a slightly lower index of refraction

– Lower attenuation, less than 15dB/km

– Lower modal dispersion, 1 ns/km or lower

– Ten times the bandwidth of step-index cable

– Most multimode fiber is graded-index cable

• Single mode fiber is a step-index cable

• Fiber designations

– Specified in terms of core, cladding and coating diameters

http://www.mun.ca/


6.4 Types of optical fiber (III)

19

Two different types of graded-index cable:

(a) Graded-index in core

(b) Graded-index in core and cladding

http://www.mun.ca/


6.5 Basic cable types

• Four broad application areas into which fiber optic cables can be classified:

– Aerial cable,

– Underground cable,

– Sub-aqueous cable

– Indoor cables

• Some specialized cables need to combine the features of several of these classes

20

http://www.mun.ca/


6.5.1 Aerial cable (I)

21

– Exposed to many external forces and hazards

– Installed between poles with the weight of the cable continuously supported by a steel messenger wire to let the cable be directly lashed, or by strength members integral to the cable construction

– Increased tensile forces are produced by the wind and ice loading

http://www.mun.ca/


6.5.1 Aerial cable (II)

– Wide variations in temperature, affecting the physical properties and attenuation of fibers

– Loose buffered fiber cable with jelly filled is predominant

– UV stabilized polyethylene is used as the cable sheath material to withstand the extremes of temperature and the intense ultraviolet light from continuous exposure to sunlight

22

http://www.mun.ca/


6.5.1 Aerial cable (III)

– Installed span length and sag requirements are important parameters affecting the maximum cable tension • Short span: less stringent tension, can be met by integral Kevlar layers

• Long span: need to utilize multiple stranded Fiber Reinforced Plastic (FRP) rods

– Advantages• Useful in areas where it may be very difficult or too expensive to bury

the cable or install it in ducts

• Also useful where temporary installations are required

– Disadvantage• Availability of system is not as high as compared to underground cables

• Storms, falling trees, flood, etc. can disrupt the communication of aerial cable

23

http://www.mun.ca/


6.5.2 Underground cable (I)– Experiences less environmental extremes than aerial cables

– Pulled into ducts or buried directly in the ground

– The cables is placed in deep narrow trenches

– Loose tube or slotted core construction

24

http://www.mun.ca/


6.5.2 Underground cable (II)

– Advantages

• The most cost effective method of installing cables outdoors

• Greater environmental protection than aerial cable

• Usually, more secure than aerial cable

– Disadvantages

• Can be disrupted by earthquakes, digging, farming, flooding, etc.

• Rodents biting, overcome by use of steel tape armor or braid, or plastic ducts, or Teflon coatings

25

http://www.mun.ca/


6.5.3 Sub-aqueous cables (I)– Basically outdoor cables

– Continuous immersion in water

– International telecommunications use the most sophisticated cables for deep ocean communications

– Along or across rivers, lakes, water races or channels

26

Distribution of submarine cable Lay out the communication cable

http://www.mun.ca/


6.5.3 Sub-aqueous cables (II)– Essentially outdoor cable constructions with a hermetically sealed

unit by using a welded metallic layer and encasing the fiber core

– Advantages• Cheaper installation in some circumstances

– Disadvantages• Unit cost of cable is higher

27

http://www.mun.ca/


6.5.4 Indoor cables (I)– Indoor cables are used inside buildings

– Have properties dictated by the fire codes

– Need to minimize the spread of fires

– Must comply with the local fire codes

– Without oil-based moisture blocking

28

Indoor soft optical cable Multi-core indoor branch optical cable

http://www.mun.ca/


6.5.4 Indoor cables (II)– Spliced to outdoor cables close to the building entry points

– Tightly buffered

– Provides adequate water resistance for indoor applications

– The buffered fibers can be given sufficient strength to enable them to be directly connected to equipment from the fiber structure without splicing to patch cords

29

http://www.mun.ca/

Chapter 6: Fiber optics overviewPractical Industrial Data Networks: Design, Installation and Troubleshooting 30

Thank you!

Any questions?

http://www.mun.ca/


Troubleshooting

Chapter 6


(II)

Feb. 17th 2011

http://www.mun.ca/


Objectives



• Splicing and connection






2

http://www.mun.ca/


How to connect fibers?

• How to connect electrical wires?

– Weld

– Twist

• How to connect electrical cables to equipments?

– BNC connector and other types of coaxial connectors

– Plug and socket connectors

– USB connectors

– Etc.

• How to connect fibers?

• How to connect fibers to optical devices?

3

http://www.mun.ca/


6.6 Connecting fibers

• Connecting fibers together and to optical devices, such as light sources and photo detectors

• Can be done using splices or connectors

• A splice is a permanent connection used to join two fibers

• A connector is used where the connection needs to be connected and disconnected repeatedly

• A device used to connect three or more fibers or devices is called a coupler

4

http://www.mun.ca/


6.6.1 Connection losses (I)

• Attenuation – the fraction of the optical power lost in the connection process, the sum of losses caused by:

– Lateral misalignment of the fiber cores

– Misalignment of the fiber axes (most important in connecting multimode fibers)

5

Lateral Misalignment

Angular Misalignment

http://www.mun.ca/


6.6.1 Connection losses (II)

– Differences in core diameters

– Core diameter mismatch occurs when the core diameter of the transmitting fiber (t) is larger than the core diameter of the fiber at the receiving end (r)

– Core diameter of Multimode fiber is 50 µm or 62.5 µm

– Single mode fiber is around 9 µm in diameter

– Diameter mismatch loss are approximated by:

Lossdia = 10 • log10 (diar/diat)2

6

http://www.mun.ca/


6.6.1 Connection losses (III)

– Numerical aperture differences of the fibers

– Define the NA of any type of fiber to be:

– n1 and n2 are refractive indices of core and cladding, respectively

– NA mismatch loss occurs when the numerical aperture of the transmitting fiber (t) is larger than that of the receiving fiber (r)

– The calculated loss for numerical aperture mismatch is approximated by:

LossNA = 10 • log10(NAr/NAt)

7

http://www.mun.ca/


6.6.1 Connection losses (IV)

– Fresnel reflection from the ends of fibers

– End separation loss

– Caused by the difference of refractive indices of two fibers

– And the spacing of the fiber ends

– Air gap will appear between two fibers

– Refractive index for air is 1, for fiber core is 1.46

– The light will be reflected back into the fiber due to a step in the refractive index on the glass-air-glass interface

8

http://www.mun.ca/


6.6.1 Connection losses (V)

– The minimum loss on the glass/air interface is about 0.35 dB

– The reflected light can damage the light source

– The use of index-matching gel in the gap can reduce Fresnel reflection loss

– End finish and cleanliness of fibers

– Physical contact finish minimizes the back reflection due to the tiny refractive index discontinuity

– Angled finish let the reflection to exit the core and dissipate in cladding

9

http://www.mun.ca/


6.6.2 Splicing fibers

• Splices are "permanent" connections between two fibers

• Splicing is only needed if the cable runs are too long for one straight pull

• Or need to mix a number of different types of cables

• Use splices to restore the broken outdoor cables

• Two types of splicing fibers

– Fusion splicing

– Mechanical splicing

10

http://www.mun.ca/


6.6.2.1 Fusion splicing (I)

– Fibers are welded together by an electric arc

– Cannot do this in an explosive atmosphere

– Factory splicing machines often use a small hydrogen flame

– Precisely pre-align the fibers

– Produce consistently lower loss splices

– Requiring expensive equipment: good fusion splicing equipments cost $15,000 to $40,000

– The splices only cost a few dollars each

– Today's single mode fusion instruments are

automated

11

Fusion splicing instrument

http://www.mun.ca/


– Fusion splicing requires stripping a longer length of bare fiber than termination

– The Miller, perhaps the most rugged, has the disadvantage of being “right-handed”

– The No-Nik is careful with the fiber but requires careful cleaning

– The Micro-Strip allows setting strip length for consistent strips

6.6.2.1 Fusion splicing (II)

12

From Left: the Miller Stripper, No-Nik and Micro-Strip

http://www.mun.ca/


6.6.2.1 Fusion splicing (III)

13

– Quality of the splice depends on the quality of the cleave

– Most splicing machines come with a recommended cleaver

– Put the bare fiber onto the V-groove

– Cover the left cap to hold the fiber

– Close the right cap and press it to clamp and tense the fiber

– Push the blade which is under the fiber to cleave it

http://www.mun.ca/


6.6.2.1 Fusion splicing (IV)

– The process of splicing preparation is the same for all splice types: strip, clean & cleave

– Repeat the process for two fibers

– Place two fibers into the guides in the fusion splicing machine and clamp them in place

– Set the splicing parameters or choose factory recommended settings

– The machine will control the splicing process itself

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6.6.2.1 Fusion splicing (V)

– The ends of the fibers are on moveable stages which are used to align the fibers and set the end gap automatically

– Optical Core Alignment (also called “Profile Alignment”): two fibers are illuminated from two directions, 90 degrees apart, software recognizes the core of the fibers and aligns them automatically using movable stages, also estimates splice loss after the fusion splicing is complete

– Local Injection and Detection

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6.6.2.1 Fusion splicing (VI)

– The splicer will show the fibers being spliced on the video screen

– Fiber ends will be inspected, and the bad one should be rejected

– Fibers will be moved into the right position through the automated splicing

– Pre-fuse cycle will remove any dirt on the fiber ends and preheat the fibers for splicing

– The fibers will be fused by an automatic arc cycle that heats them in an electric arc and feeds the fibers together at a controlled rate

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6.6.2.1 Fusion splicing (VII)– Splicing machines generally have a

heating device for heat shrinking a protective sleeve over the finished splice

– The sleeve is made of plastic with a metal rod inside

– It can protect fiber from moisture or other environmental hazards

– One fiber should pass through the sleeve before splicing

– Sleeves have a standard length

– Put the sleeve into the heat device and close the transparent cover

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6.6.2.2 Mechanical splicing (I)

– Fibers are held together in an alignment structure

– Using an adhesive (index matching gel or glue) or mechanical pressure

– Rely on aligning the outer diameters of the fiber cladding

– Little glass tubes, V-grooves, sleeves, 3-rods and various proprietary clamping structures

– Low price for the tools to make mechanical splices

– Have a high consumable cost per splice

– Work well with both single mode and multimode fiber

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6.6.2.2 Mechanical splicing (II)

– After preparation, the fibers are inserted into the splicing element. The assembly tool is then used to close the cap, forcing the clamping and locating surfaces against the fibers and aligning the fibers precisely and permanently in place.

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3M Fibrlok™ II Universal Optical Fiber Splice 2529

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6.6.2.3 Choose the splicing

– Based on cost and location

– Fusion is expensive equipment and cheap splices

– Mechanical is cheap equipment and expensive splices

– If make a lot of splices (thousands in an big telecommunication or CATV network), use fusion splices

– If needs just a few splices, use mechanical splices

– Fusion splices give very low back reflections and are preferred for single mode high speed digital or CATV networks

– Mechanical splices are preferred for multimode fibers

– If it is an underwater or aerial application, the greater reliability of the fusion splice is preferred

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6.6.3 Connector

• Make flexible interconnections between optical devices

• Repeatedly align the fibers

• Have significantly greater losses than splices

• Main loss is the axial misalignment of the fibers

• Connector loss is in the range from 0.2 to over 3 db

• Butt joint with the fiber ends

• The fiber is mounted in a ferrule made of metal or ceramic

• The central hole of ferrule matches the fiber cladding diameter

• Fiber is glued into the ferrule, the end is cut and polished to be flush with the face of the ferrule

• Types of connectors: SC, ST, etc.

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6.6.3.1 SC connector (I)– Subscriber Connector (SC), a general purpose push/pull style Connector

developed by NTT, Japan

– SC is a snap-in connector that is widely used in single mode systems for it's excellent performance

– Built with a cylindrical ceramic ferrule

– Square cross-section for high packing density

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SC connector SC connector SC adapter

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6.6.3.1 SC connector (II)– Push–pull latching mechanism

– SC connectors offer low cost, simplicity, and durability

– SC connectors provide for accurate alignment

– Specified loss of less than 0.6 db (typically 0.3 db)

– Typical return loss of 45 dB

– It is also available in a duplex configuration

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6.6.3.2 ST connector (I)– Straight Tip connector

– An older standard used for data communications

– ST is the most popular connector for multimode networks, like most buildings and campuses

– Has a round cross-section

– It has a bayonet mount and a long cylindrical ferrule to hold the fiber

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ST connector ST connector and adapters

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6.6.3.2 ST connector (II)

– Most ferrules are ceramic, but some are metal or plastic

– Relies on the internal spring to hold the ferrules together

– Have to make sure connectors are seated properly

– If it has high connection loss, reconnect them

– Tensile strength is less than 1 kg force

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ST connectorAdaptor of ST connector

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6.6.3.3 FC connector (I)

– The ferrule connector (FC) is a sort of fiber-optic connector with a threaded body

– The FC connector is designed for use in high-vibration environments

– It is commonly used with both single-mode optical fiber and polarization-maintaining optical fiber

– The FC Connector is the most popular connector used today

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6.6.3.3 FC connector (II)

– The fiber end is embedded in a 2.5 mm ferrule made of ceramic or stainless steel

– The tip is typically polished to produce a rounded surface, called "physical contact" (PC) polish

– Different designed: FC/PC (FC/SPC, FC/UPC), FC/APC (Angled Physical Contact)

– The angle of FC/APC ferrule is 8 degree

– The Insertion Loss for matched FC connectors is 0.25 dB

– The FC screws on firmly, but make sure the “Key” has been aligned in the slot properly before tightening

– How to distinguish FC/PC and FC/APC?

– The color of boot: blue or black for FC/PC, green for FC/APC

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6.6.3.4 Other types of connectors (I)

– Small Form Factor (SFF) connectors for high density connection

– LC is a new connector that uses a 1.25 mm ceramic ferrule, half the size of the ST

– The ferrule connector is easily terminated with any adhesive

– Good performance, highly favored for single mode

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6.6.3.4 Other types of connectors (II)

– MT-RJ is a duplex connector with both fibers in a single polymer ferrule

– It uses pins for alignment and has male and female versions

– Multimode only, field terminated only by pre-polished/splice method

– The MT-RJ connector has become the most common SFF connector

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6.6.3.4 Other types of connectors (III)– Opti-Jack is a neat, rugged duplex connector cleverly designed as

two ST-type ferrules in a package with the size of a RJ-45, has male and female (plug and jack) versions

– E2000/LX-5 is like a LC but with a shutter over the end of the fiber

– MU looks a miniature SC with a 1.25 mm ferrule, popular in Japan

– MT is a 12 fiber connector for ribbon cable, main use is for pre-terminated cable assemblies

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Opti-Jack E2000/LX-5 MU connector MT connector

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6.6.3.5 Structure of connectors (I)

– The fiber ferrule: is built around a long cylinder with 2.5mm outer diameter, 124--127µm inner diameter

– The connector sub-assembly body: the ferrule is assembled in it, has mechanisms to hold the cable and fiber in place

– The connector housing : assembles with sub-assembly body and provides the mechanism for snapping into a mating sleeve (adapter) and hold the connector in place

– The fiber cable: are crimped onto the connector sub-assembly body with strength member (Kevlar) by a crimp eyelet

– The stress relief boot: covers the joint between connector body and fiber cable, protects fiber cable from mechanical damage

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6.6.3.5 Structure of connectors (II)

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SC Connector Structure

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6.6.3.6 Ferrules (I)

– The ferrule is the most important and costly part of a fiber connector

– Materials of ferrules include ceramic, plastic or stainless steel

– Holds the end of the fiber and precisely aligns it to the socket

– The fiber is inserted into the ferrule and cemented with an epoxy or adhesive

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6.6.3.6 Ferrules (II)

– Connectors may also use crimped ferrules that do not require cement

– Give fiber long-term mechanical strength and prevent the contamination from environment

– If the length, hole centering, inside and outside diameters are not exact, a poor connection will result

– 2.5mm ferrule for standard fiber optic connectors: SC, ST, FC, Opti-Jack connector, etc.

– 1.25 mm ferrule for small form factor fiber optic connectors: LC, MU, etc.

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6.6.4 Connector handling (I)

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• Most fiber optic connectors are designed for indoor use

• Connectors for outdoor use require to be hermetically sealed to protect the optical faces from contamination

• Do NOT touch the end of the connector ferrules

Assemble a ST fiber optic connector

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6.6.4 Connector handling (II)

• Clean connectors with lint-free wipes and isopropyl alcohol

• Compressed air sprays are available for cleaning connectors and adapters without needing to physically touch the mating surfaces

• Indoor fiber connectors are 500 to 1000 mating cycles

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fiber connector/adapter cleaning kit Compressed air

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6.6.5 Assemble a fiber connector (I)

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Thorlabs Fiber Optic Assembly Parts

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6.6.5 Assemble a fiber connector (II)

– Step 1: Clean Fiber Optic Connectors, use the isopropyl alcohol to completely cover the connectors to remove contaminations

– Step 2: Add Strain Relief Boots and Crimp Sleeves

– Step 3: Prepare Furcation Tubing

– Step 4: Inserting the Optical Fiber into the

Furcation Tubing

– Step 5: Trim and Strip Fiber to Length

– Step 6: Clean Optical Fiber

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6.6.5 Assemble a fiber connector (III)

– Step 7: Add Epoxy to the Connector

– Step 8: Insert Fiber into Connector

– Step 9: Fully Seat Connector

– Step 10: Secure Crimp Sleeve

– Step 11: Epoxy Curing

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6.6.5 Assemble a fiber connector (IV)

• Fiber Polishing

– Step 1: Scoring the Fiber

– Step 2: Cleaving the Scored Fiber

– Step 3: Assemble the Polishing Disc and Connector

– Step 4: 5 μm Polish, 3 μm Polish, 1 μm Polish, 0.3 μm Polish

– Step 5: Final Inspection

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6.6.6 Optical couplers

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• Optical couplers or splitters and combiners are used to connect three or more fibers or other optical devices

• Split the input power to a number of outputs

• Coupler configuration depends on the number of ports and whether each of these are unidirectional

1:2 optical coupler Optical coupler

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6.7 Splicing trays/organizers and termination cabinets

• Different types of storage units that are used for housing optical fiber splices and end of cable terminations

• Splicing trays

• Splicing enclosures

• Termination in patch panels and distribution frames

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6.7.1 Splicing trays (I)

• Splices are generally located in units

• Referred to as ’splicing centers’, ‘splicing trays’ or ‘splicing organizers’

• Provide a convenient location to store and to protect the cable and the splices

• Provide cable strain relief to the splices themselves

• Located at intermediate points along a route where cables are required to be joined

• Locates at the termination and patch panel points at the end of the cable runs

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6.7.1 Splicing trays (II)

• Sheath of the input cable is stripped away and brought into the splicing center

• The fibers are looped completely around the tray and into a splice holder

• The fibers are spliced onto the outgoing cable if it is an intermediate point

• Or on to pigtails if it is a termination point

• Looped completely around the tray and then fed out of the tray

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6.7.1 Splicing trays (III)

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A typical splicing tray

A example of a splicing tray

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6.7.1 Splicing trays (IV)

• Cables are physically attached to the splice tray to provide strain relief

• Fibers are looped completely around the tray to provide slack and tension relief

• Each splice joint is encased in a splice protector (plastic tube) or in heat shrink before it is clipped into the holder

• Allows different fibers to be cross-connected and to be looped back for testing purposes

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6.7.2 Splicing enclosures (I)

• The splicing trays are not designed to be left in the open environment and must be placed in some type of enclosure

• Direct buried cylinders

– Two cables are joined to continue a cable run

– Placing the splice trays in a tightly sealed cylindrical enclosure

– Enclosure is made from heavy duty plastic or aluminum

– Container is completely sealed from moisture ingress and contains desiccant packs to remove any moisture that may get in

– Cables normally enter the enclosure at one end only to allow the enclosure to be lifted from the ground for easier splicing access

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6.7.2 Splicing enclosures (II)

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Direct buried cylinders

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6.7.2 Splicing enclosures (III)

• Termination cabinets

– a lot of cables

– splicing trays are stored in a larger wall mounted cabinet

– approximately 500 mm × 500 mm ×100 mm

– For outdoor use, the cabinets must be sealed against bad weather conditions

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6.7.2 Splicing enclosures (IV)

• Patch panels and distribution frames

– Installed in the back of patch panels and distribution frame

– For connection of patch cords to the main incoming cable

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6.7.3 Termination in patch panels and distribution frames (I)

• Three main methods of connecting an incoming cable into a patch panel or distribution frame

• Firstly:

– Fibers of incoming cable have a large minimum bending radius

– Splice each fiber to a fiber patch cord with a smaller bending radius

– Reduces undue stress on the incoming cable

– Introduce small losses into the link

– Replaces the more fragile glass of the incoming cable with more flexible and stronger glass of the patch cords

– Termination cabinet for splicing trays

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6.7.3 Termination in patch panels and distribution frames (II)

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• Secondly:

– Place the fibers from the incoming cable into a breakout unit

– Breakout unit separates the fibers and allows a plastic tube to be fitted over the incoming fibers to provide protection and strength as they are fed to the front of the patch panel

– There are no splices keep losses to a minimum

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6.7.3 Termination in patch panels and distribution frames (III)

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• Third method:

– Tight buffered fibers in incoming cable are flexible and strong

– Can be taken directly to the front of the patch panel

– Referred to as direct termination

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Thank you!

Any questions?

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Troubleshooting

Chapter 6


(III)

Feb. 24th 2011

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Objectives



• Splicing






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6.8 Troubleshooting

• 6.8.1 Introduction

– Deal with problems on optical fiber cables

– Problems can be caused by poor installation practices

– Fibers are subjected to excessive tension or bending forces

– Basic methods of testing fibers

– How to locate faults on fiber optic systems

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6.8.2 Standard troubleshooting approach (I)

4

• The standard approach for troubleshooting fiber optic systems:

1. Observe the system status indicators and determine whether signals are being transmitted and received at both terminals

2. Determine whether the appropriate fibers are functional by either a continuity test or a qualitative insertion loss measurement between the patch panels

3. Once the faulty fiber is identified, clean the optical connectors and repeat the test

4. If the fault remains, swap the system onto a spare fiber by rearranging the patch cords on the patch panels at both ends

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6.8.2 Standard troubleshooting approach (II)

5

• The standard approach to troubleshooting fiber optic systems:

5. Update the records to indicate the faulty fiber

6. When a link does not have sufficient spare fibers available to maintain system integrity, then attempt fault localization using an OTDR

7. On short lengths of fiber, total replacement may be more cost effective than expensive location and subsequent repair. This is particularly appropriate where a spare duct is available for the cable replacement

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6.8.3 Tools required (I)

6

• Continuity tester

– A fiber optic transmitter with a suitable fiber optic connector

– Transmits a visible red light @ 650 nm

– Transmits visible light over several kilometers

– Finding fractures in fibers or bad splices by observing leaking out light and for identifying fibers at the end of a cable

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6.8.3 Tools required (II)• Optical source

– High intensity light emitting diodes (LEDs) or visible lasers

– Visible lasers produce a more powerful light and can provide more information to the technician than a LED

– By coupling the fiber to a visible laser, problems with connectors, breaks in the fiber near connectors or splices can be located

– Has a calibrated fiber optic transmitter

– With an optical power meter for insertion loss testing of fibers

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LED Laser

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6.8.3 Tools required (III)• Optical power meter

– A fiber optic receiver with a suitable fiber optic connector

– Displays the received optical power levels

– Used with an optical source for insertion loss testing of fibers

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6.8.3 Tools required (IV)• Optical time domain reflectometer

(OTDR)

– Sends a short pulse of light down the fiber

– Measures and records the light energy reflected back up the fiber

– A reflection may be caused by the presence of a connector, splice, crack, impurity or break in the fiber

– Calculate the distance to the reflection point by measuring the reflection time and knowing the refractive index of the fiber

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6.8.3 Tools required (V)

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6.8.4 Fiber installation rules

11

• General installation rules for installing fiber optic cabling systems

• Avoid long-term reliability problems

• Fibers break at any surface defects if subjected to excessive bending forces while under tension

• Cable bending radius

• Cable tension

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6.8.4.1 Cable bending radius (I)

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– The cable radius is not less than the minimum installation bending radius, 50 mm for standard fiber

– Avoid sharp bends along the installation route

– Ensure that the conduit or the cable tray is constructed with no sharp edges, use curved construction components

– Ensure the cables are laid on to a flat surface, no heavy objects will be laid on to the cables in the future

Bend radius:

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6.8.4.1 Cable bending radius (II)

– Avoid putting kinks or twists into the cable, pulling the cable directly

– A minimum bending radius applies to the long-term installed cable

– Macro-bends in the fibers are caused by sharp bends which lead to fiber breakage and additional signal attenuation

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Macro-bend losses:

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6.8.4.2 Cable tension (I)

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– Surface defects and micro-cracks on a optical fiber will be caused by a longitudinal tensile force

– Optical fibers have some elasticity, the fiber may theoretically stretch as much as 9% before breaking

– In practice, to permanent strains of less than 0.2% to avoid promoting premature failures

– The maximum permissible installation cable tension is specified by the manufacturer

– Avoid sudden, short sharp jerking, should be pulled in an easy smooth process

– When pulling cable off a large drum, ensure that the cable drum is smoothly rotated by one team member to feed off the cable

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6.8.4.2 Cable tension (II)

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– To minimize cable stress after the installation is complete

– A slack final resting condition will help to ensure a long operating life of the fiber optic cable

– Slack is left in the junction boxes at the completion of the installation to reduce overall stress in the cable

– Use intermediate junction boxes to reduce cable tension when a lot of bends in the cable route

– The cable can be pulled through at these points, laid out in a large figure ‘8’ pattern on the ground and then pulled into the next section to avoid kinking and twisting of the cable

– Curved guides or block systems may be used in the junction boxes where the cable changes direction

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6.8.5 Clean optical connectors (I)

16

• Dust particles and other particles can cover the core area of fiber and may damage the connector

• MUST clean connectors completely

• Wipe the end face and ferrule of the connector with some isopropyl alcohol on a lint free tissue

• Cleaning tissues is obtained in sealed packages already impregnated with alcohol

http://www.mun.ca/http://www.thorlabs.com/images/TabImages/FiberCleaningWipeHand-780pix.jpghttp://www.thorlabs.com/images/TabImages/Step2-780pix.jpghttp://www.thorlabs.com/images/TabImages/Step3-780pix.jpghttp://www.thorlabs.com/images/TabImages/Step4-780pix.jpg


6.8.5 Clean optical connectors (II)

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• Cassette type cleaners, a dry tape which is advanced every time the cassette is opened

• Use a can of compressed gas to clean adapters and bare fibers

• Fit dust caps to all unused patch panel ports

Cassette type cleaners Use compressed gas Dust caps of fiber adapters

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6.8.6 Locating broken fibers

18

• 6.8.6.1 Continuity testing

– Continuity test simply checks that the fiber is continuous from one end to the other

– Shining a powerful torch beam (or laser pointer) into one end of the fiber and observing the light coming out of the other end

– Cost effective method for multimode fibers rather than single mode fiber since the difficulty of light coupling into the fiber

– 650 nm visible red light to find fractures in fibers or bad splices

– Limited application, as it is of no use in finding faults in buried cables or aerial cables

– User should not look into fiber groups at the end of cables if any fibers on the system at any location are connected to lasers

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6.8.6.2 Insertion loss testing (I)– Measure the attenuation of the length of fiber to qualify a fiber

optic system

– Carried out with a power source and a power meter

– The power meter is calibrated to the power source by connecting a short piece of optic fiber approximately 2 m in length

– The power source is set to transmit a level of –10 dBm (0.1mW) and the power meter then adjusted accordingly to read –10 dBm

– Same optic fiber type between calibrated fiber and tested fiber

– Same wavelength as the installed system equipment

– Same source and detector types

– Same connectors are used for calibration as are used in the installation

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6.8.6.2 Insertion loss testing (II)

– The insertion loss through the cable section caused by the optic fiber, splices and the connectors

– Attenuation (dB) = −10 log (PO/PI)• PO is power out of the fiber (in mW)

• PI is power into the fiber (in mW)

– Insertion loss measurement be performed in both directions of an installed cable

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6.8.6.3 Optical time domain reflectometer (I)

– Optical time domain reflectometer (OTDR)

– The only method of analyzing the losses along an individual fiber

– Sends a short pulse of light down the fiber and measures and records the light energy that is reflected back up the fiber

– A reflection may be caused by the presence of a connector, splice, crack, impurity or break in the fiber

– Calculate the distance to the reflection point by measuring the time, it takes for the reflected light to return to the source and knowing the refractive index of the fiber

– Impurities in the glass will cause continuous low level reflection as the light travels through the glass fiber, due to Rayleigh scattering (backscatter)

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6.8.6.3 Optical time domain reflectometer (II)– The strength of the backscattered signal received at the source

gradually drops as the pulse moves away from the source. This is seen on an OTDR display as a near linear drop in the received reflected signal and the slope of this line is the attenuation of the fiber (dB per km)

– Do not provide accurate readings of irregularities and losses in the fiber for the first 15 m of the cable

– The pulse length and its rise time from the OTDR are comparatively large when compared to the time it takes for the pulse to travel the short distance to the point of reflection within this 15 m and back

– For shorter local cable running less than 200 m, there is not a lot to be gained from carrying out an OTDR test unless there are connectors and splices along the cable route

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6.8.6.3 Optical time domain reflectometer (III)

23

– The y-axis of the plot shows the Relative amplitude of the light signal that has reflected back to the source and the x-axis Represents time. The time base is directly translated and displayed as distance by the OTDR

– The sudden peaks that appear along the slope are the points where reflections have occurred and light that has reflected back to the source is stronger than the backscatter

Point (6) illustrates a splice where the cores of the fibers are well matched for light traveling in the direction away from the source

Point (7) shows the noise floor of the instrument. This is the lowest sensitivity of received signal that the device can accept

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6.8.6.3 Optical time domain reflectometer (IV)

• Four main reflection points in their order of decreasing magnitude

1. Reflection from the un-terminated end of the fiber

2. Reflection from a connector

3. Reflection from a splice

4. Reflection from a hairline crack in the fiber

5. Backscatter

• After each of the reflections, the slope of the attenuation curve drops suddenly

• Represents the loss introduced by the connector, splice or imperfection in the fiber

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6.8.6.3 Optical time domain reflectometer (V)

25

• OTDR testing can provide very accurate fault analysis over long lengths of fiber

• A resolution of 1 m for fault location and .01 db for in line losses is available

• Operate with a range up to 200 km

• Relatively easy to use and special analysis software packages are available for downloading the test results and carrying out detailed analysis if required

• Unfortunate downside with OTDR technology is that it is generally very expensive

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6.8.6.3 Optical time domain reflectometer (VI)

• Different fibers are joined together, this connection may represent a change in refractive index, core size, modal properties and/or material properties of the fiber

• The OTDR test should be carried out on every optical fiber in a cable while it is still on the reel prior to installation to ensure that faulty fibers are not installed

• Once the cable has been installed, the OTDR tests should be carried out again on every optical fiber

• The results of the pre-installation and the post-installation tests should be kept as part of the commissioning documentation

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6.8.6.3 Optical time domain reflectometer (VII)

• OTDR measurements are a relative measurement rather than an absolute measurement

• The wavelength at which OTDR operates is not important

• The loss of a connector or splice might be different when measured from each direction into the optic fiber

• The length of the cable is not going to be the length of the fibers for the case of some fiber optic cables are constructed so that the fibers are laid in a helical fashion around the center of the cable

• Difficult to determine the distance to faults

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6.8.6.3 Optical time domain reflectometer (VIII)

• Manufacturer will generally provide a ratio of fiber length to cable length

• Used to calculate the exact cable distance to the fault from the OTDR distance reading

• An OTDR measurement is performed on a known length of cable (generally 1 km) and the ratio is calculated as:

28

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Thank you!

Any questions?

http://www.mun.ca/

DAC - Digital to Analog ConversionADC - Analog to Digital ConversionRS485 - Modbus

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SCADA - Supervisory Control And Data Acquisitionmasek/7680_notes.pdf · 2017. 1. 25. · SCADA - Supervisory Control And Data Acquisition ENGI-7680 Winter 2016. DAC ... The process